1. Field of the Invention
The present invention relates to a semiconductor memory device in which data values are sensed by current-to-voltage conversion, more particularly to the provision of a suitable reference current and reference voltage in this type of memory device.
2. Description of the Background Art
Current signals are used for data transport in, for example, non-volatile memory devices with single-transistor memory cells. When selected, the memory cells either conduct or block current, depending on the stored data value. Sensing circuits in the memory device convert the current flow to a data voltage signal Vs that has a high level or a low level, depending on the stored data value.
Such a memory device also has reference memory cells that always conduct current when selected. The sensing circuits convert a reference current received from the reference memory cells to a reference voltage signal Vref having a level intermediate between the high and low Vs levels. The difference between Vs and Vref is amplified to produce a `1` or `0` data output signal.
For quick and reliable amplification of the Vs-Vref difference, the reference voltage level Vref is preferably disposed halfway between the two Vs levels. In conventional memory devices, however, depending on the location of the memory cell being read, or on the data stored in other memory cells, the Vref level may be quite close to one of the two Vs levels. A resulting problem is that extra time must be allowed for amplification of the Vs-Vref difference, so the stored data cannot be accessed quickly. In the worst case, the wrong data value may be read.
Further explanation of this problem will be given in the detailed description of the invention.